A typical cellular radio access system includes a radio access network (RAN) that is arranged to provide mobile stations with access to one or more transport networks such as the public switched telephone network (PSTN) and/or the Internet. In an example, the RAN includes a cellular base station (e.g., base transceiver station, access node, eNodeB, or the like), which may include an antenna configuration and associated equipment for radiating to define one or more coverage areas in which a mobile station can wirelessly communicate with the base station over an air interface. The base station may operate according to a defined air-interface protocol such as CDMA, LTE, WiMAX, and/or GSM, and the mobile station may operate according to the same air-interface protocol to enable and facilitate communicating with the base station and gaining access to one or more transport networks via the RAN.
The cellular radio access system may further include various entities such as switches, gateways, and controllers that may facilitate connectivity with a transport network and/or may help control aspects of RAN operation such as registration and de-registration of mobile stations. In an example, the cellular radio access system may include a mobile switching center (MSC), a mobility management entity (MME), a soft-switch, a media gateway controller (MGC), a packet data serving node (PDSN), a serving gateway (S-GW), and a packet data network gateway (P-GW). Further, these entities may be coupled with a signaling network to facilitate communication with other system entities such as a home location register (HLR) (e.g., a home subscriber server (HSS)) or a location-determination system, among other entities.
An HLR may function to store service-profile records for mobile stations that subscribe to service from the cellular radio access system. For each respective mobile station, the service-profile record (e.g., a UE context) may indicate services that the RAN is to provide for the mobile station. Further, the HLR may keep track of which portion of the cellular radio access system is serving the mobile station at any given time, so as to facilitate routing of calls or other communications to the mobile station. For instance, the HLR may maintain for each mobile station a record of the base station, switch, gateway or other node currently serving the mobile station, or last known to be serving the mobile station. When an entity seeks to route a communication to the mobile station, the entity may thus query the HLR to identify the base station that is currently serving the mobile station and may then route the communication to that base station for wireless transmission to the mobile station over an air interface.
Base stations in the cellular radio access system may be arranged to broadcast in each of their coverage areas a respective pilot signal (or reference signal), which may enable mobile stations to select an appropriate coverage area in which to operate. The mobile stations may correspondingly scan for these pilot signals and attempt to register in a coverage area that is broadcasting a sufficiently strong signal.
In practice, when a mobile station enters into coverage of the RAN, such as coverage of a base station, the mobile station may engage in a registration (e.g., attach) procedure so as to then be able to engage in bearer-data communication, perhaps to place and receive calls and engage in wireless packet-data communication, to the extent that the mobile station's service profile and capabilities allow, perhaps. During the registration procedure, the mobile station may transmit a radio-access-registration-request (e.g., attach-request) message via an air-interface uplink channel to the base station providing the coverage area. Upon receiving that registration-request message, the base station may signal to one or more other entities of the cellular radio access system, such as an MSC or MME as examples, which in turn may signal to the HLR.
As a result of this signaling, the mobile station may be authenticated and its service subscription verified, and the HLR may receive an update (from the base station or other entity) indicating where in the network the mobile station is located. Further, one or more entities may facilitate the setup of a bearer-data communication channel for the mobile station, and the HLR may provide the base station, MSC, MME, or the like with a copy of the mobile station's service profile for local storage, which may then be used to facilitate serving the mobile station. The base station may then transmit to the mobile station over an air interface downlink channel a registration-response (e.g., attached-accept) message, acknowledging the registration. Once registered, the mobile station may engage in bearer-data communication in accordance with the mobile station's service profile and capabilities.
After initially registering in a coverage area, a mobile station may continue to monitor the pilot signal of that coverage area as well as the pilot signals of adjacent coverage areas, and may at some point transmit to the serving base station a radio measurement report (e.g., pilot-strength-measurement message or data-rate-control message) indicating the strength of the monitored signals. If the base station determines based on such a message that an adjacent coverage area provides sufficiently stronger coverage than the currently serving coverage area, the base station may arrange for a handoff of the mobile station to the adjacent coverage area.
To initiate a handoff of a mobile station to a target base station, a source base station may send a handover-request message to the target base station, which in turn may accept the handoff request by sending a handover-request-acknowledgment message to the source base station. Upon receiving the handover-request-acknowledgment message, the source base station may send to the mobile station a handoff-direction message, indicating to the mobile station that subsequent air-interface communication should be conducted via the target-base-station coverage area. The source base station may also send to the target base station via an inter-base-station link (e.g., an X2 link) between the source and target base stations a locally-stored service profile of the mobile station, which the target base station may require before serving the mobile station. Further, the source base station may forward downstream bearer data to the target base station via the inter-base-station link until the source base station receives a release-resource message (described below) from the target base station. The forwarded data can then be sent to the mobile station once it begins operating on the target-base-station coverage area.
Upon receiving a handover-complete message from the mobile station and receiving the service profile from the source base station, the target base station may send a path-switch-request message to an S-GW to request that downstream bearer data destined for the mobile station be directed to the target base station (rather than to the source base station). The target base station, upon receiving a path-switch-request-acknowledgement message from the S-GW, indicating that downstream bearer data has been redirected to the target base station, may then send a resource-release message to the source base station, indicating that the source base station may release any resources still allocated to the mobile station.